The energy that the human body burns and the amount of food consumed determine a person's body weight. If food intake covers the amount of energy burned, body weight remains constant - a state known as energy balance. Achieving an energy balance is not easy in everyday life. This is reflected in the increasing number of people suffering from morbid obesity. To counteract this development, it is important to have a better understanding of how much food a person should eat.

In this study, the investigators will investigate the amount of food needed to meet a person's energy needs and bring them into energy balance.

Primary aims of the study are i) to technically and biologically validate two whole-room indirect calorimeters (WRICs) and ii) by using whole-room indirect calorimetry, to achieve a more accurate estimate of a person's emergy balance compared to common approximation formulas.

Secondary study aims:

1. To investigate whether the transfer of a person into energy balance using WRIC has an influence on energy expenditure measures compared to the transfer into energy balance using the usual approximation formula.
2. To investigate whether the transfer of a person into energy balance using WRIC has an influence on activity-dependent energy expenditure measures compared to the transfer into energy balance using the usual approximation formula.
3. To investigate whether differences in energy expenditure during energy balance during moderate and strenuous physical activity influence food intake.
4. To investigate whether energy intake in relation to energy expenditure during energy balance is related to weight development

Technical Validation: Eight 24-hour methanol burns within each of the two WRICs will be conducted for technical validation.

Biological Validation and Energy Balance Component:

After providing informed consent and meeting inclusion/exclusion criteria, study participants will follow a weight-maintaining diet before their first 24-hour stay in a WRIC (Day 4). During this stay, energy expenditure will be measured under conditions of estimated energy balance. As a measure of energy expnediture relative to energy intake, energy balance will be estimated based on resting energy expenditure, an estimated physical activity level, and an approximation formula applied during the screening procedure.

A second 24-hour stay in the WRIC (Day 6), also under conditions of estimated energy balance, will serve for biological validation. After following the weight-maintaining diet for three additional days, a third 24-hour WRIC stay (Day 10) will assess the achievement of near-perfect energy balance. On the day following this stay (Day 11), participants will have access to a buffet and will be allowed to eat ad libitum.

Subsequently, a 3-day run-in period under weight-maintenance conditions will precede reassessment of 24-hour energy expenditure during increased physical activity in the WRIC (Days 15 and 17). Energy expenditure during energy balance will again be compared to ad libitum food intake at a buffet (Day 18).

Follow-up weight measurements will be conducted one year after study completion to evaluate the influence of achieving near-perfect energy balance-or deviations from it-on weight change. Additionally, fasting blood samples will be collected before and after each WRIC stay, as well as before and after ad libitum food intake, to measure hormones related to appetite control and satiety.

N = 34 subjects are required to detect a statistically significant difference in energy balance after measuring energy expenditure using a WRIC. However, interim analysis after n = 8 subjects will be conducted to adjust needed sample size to variability as detected using here implied methods of indirect calorimetry.

The statistical evaluation with regard to i) technical validation, ii) biological validation, and iii) testing the achievability of an energy balance and its effect on energy consumption are carried out using parametric and non-parametric tests. Data are given as mean value with standard deviation. The significance level is P < 0.05.

Inclusion Criteria:

• Abilty to provide informed consent
• Written declaration of consent
• Healthy study participants
• Able to perform moderate physical exercise using a bike ergometer
• Women: continuous contraception/fullicular phase of menstrual cycle

Exclusion Criteria:

• Weight change > 5 kg or 5% of body weight in the last 3 months
• Nicotine abuse, Alcohol/drug abuse
• Strenuous physical activity in everyday life > 1 h per day
• Body mass index < 18.5 kg/m² or ≥ 40 kg/m²
• Chronic diseases with an impact on energy expenditure
• Food allergy/intolerance, vegan diet
• Circumstances that speak against the application of wearable accelerometers (e.g. silicone contact allergy)
• Impaired fasting glucose, diabetes mellitus and prediabetes
• Pregnancy/breastfeeding
• Claustrophobia
• Refusal to communicate incidental findings

Cisgender subjects

Healthy, non-obese subjects of both sexes.

The decision not to share IPD is based on considerations of participant privacy and data protection. Although all data are anonymized, the sensitive nature of the information collected in this study necessitates careful handling to minimize any risk of identification. Furthermore, the informed consent provided by participants does not explicitly include provisions for data sharing beyond the current study.

To ensure compliance with ethical guidelines and data protection regulations, the study team has decided not to make IPD available publicly or to external researchers. However, summary data and study findings will be shared through peer-reviewed publications, ensuring transparency and the opportunity for scientific discourse.